Stabilizing Fly Ash in Epoxy Composites via Reaction‐Induced Phase Separation: Interfacial Engineering for High‐Performance Multifunctional Materials

ABSTRACT This study addresses the challenge of fly ash (FA) sedimentation and poor dispersion in epoxy resin (EP) composites, which severely limits their performance and high‐value utilization. To overcome this, we propose a synergistic multiphase strategy by co‐incorporating polyether sulfone (PES) and FA into the diglycidyl ether of bisphenol A (DGEBA)/4,4′‐diaminodiphenylsulfone (DDS) system, employing reaction‐induced phase separation to construct a stable EP/PES interfacial structure. This approach effectively suppresses FA settling while enabling precise morphological control and performance enhancement. The results show that PES addition significantly mitigates FA deposition and regulates the composite's phase morphology. An optimized composite containing 15 wt% PES and 50 wt% FA 2 ( d < 35.44 μm) exhibits a sea‐island structure and outstanding comprehensive properties: a tensile strength of 79.57 MPa, a bending strength of 190.58 MPa, an impact strength of 22.05 kJ/m 2 , and a low thermal conductivity of 0.101 W/(m·K), along with excellent flame retardancy (LOI = 30.0%, no dripping). The synergistic system demonstrates markedly superior mechanical performance compared to systems containing only PES (e.g., 15% PES: 74.24 MPa, 109.50 MPa, 18.94 kJ/m 2 ) or only FA (e.g., 50% FA 2 : 49.92 MPa, 110.81 MPa, 21.17 kJ/m 2 ). This work elucidates the cooperative mechanism of PES and FA in simultaneously reinforcing, toughening, and functionalizing EP composites, providing an effective strategy for the high‐value recycling of industrial solid waste (fly ash) and offering valuable insights for the design of advanced multifunctional polymer composites.